1. Field of the Invention
The present invention relates to a chip antenna used for an electronic apparatus for performing wireless communications, such as a mobile communication device, a personal computer, and the like.
2. Background Art
Among portable terminals, such as cellular phones, there is a continuous rise in the number of devices, each equipped with a chip antenna for performing wireless communications with other electronic devices, in addition to an ordinary whip antenna or a built-in antenna used for the purpose of telephone communications.
There is also an increase in the number of handy mobile electronic devices, such as notebook type computers, capable of performing wireless data communications with a growing number of these electronic devices being equipped with chip antennas.
In addition, miniaturization of the chip antennas is strongly desired, since downsizing and energy-saving features are the essential requirements for portable terminals and notebook type computers in recent years. It is further desired that the chip antennas are capable of performing transmission and reception of various bands of frequencies because they need to be adaptable for communications according to a plurality of standards due to the recent diversification of communication services.
As an example of the above chip antennas, there is one kind which comprises a helical conductor provided on an insulating substrate of a prismal shape and terminals at both ends, wherein one of these terminals is used for a power receiving connection. (Refer to Japanese Patent Laid-open Publication, No. 2001-326522, for example.) FIG. 44 is a perspective view depicting a chip antenna of the prior art. Substrate 103 is constructed of a prism-shaped insulating material such as ceramic, for instance, and power is supplied to one of terminals 101 and 102 which are provided at opposite ends of the substrate 103. Helical conductor 104 is formed by winding a copper wire or the like, or by trimming a conductive layer plated on substrate 103. Chip antennas of this kind can be mounted easily into portable terminals and the like because they can be made very small.
There is another type of antenna, a single unit which alone is capable of transmitting and receiving signals of a plurality of frequencies. (Refer to Japanese Patent Laid-open Publication, No. 2002-33616, for example.) Use of an antenna of this kind makes it unnecessary for the portable terminal to have a plurality of antennas, since it can transmit and receive radio waves of a plurality of frequencies with the single antenna.
However, the chip antennas disclosed in the Publication, No. 2001-326522 can transmit and receive only radio waves of a single frequency, although they are very small in size.
On the other hand, the chip antennas disclosed in the Publication, No. 2002-33616 have a comparatively large construction and are not suitable for downsizing because of the complex structure requiring a number of components and power feeding elements, although they can transmit and receive radio waves of the plurality of frequencies. When consideration is given, especially to the processes up to actual mounting, it becomes apparent that downsizing is quite difficult. In particular, the chip antennas need to be adaptable for downsizing, low-profiling and energy-saving for the portable terminals, notebook computers, and the like.
Furthermore, since downsizing and energy-saving features are essential for cellular phones and notebook computers these days, it is desirable to miniaturize the antenna devices. It is also desirable that the antennas are capable of working on a wide frequency band, as the transmission capacity increases. In addition, a further increase in the operable bandwidth is necessary for the multi-career methods such as OFDM (“Orthogonal Frequency Division Multiplexing”). A small, light-weight chip antenna capable of working on a wide frequency band can be made possible by adding a conductor to a top end portion of the chip antenna to form a capacitance. (Refer to Japanese Patent Laid-open Publication, No. H10-242731, for example)
FIG. 45 is a perspective view of a chip antenna of the prior art provided with a conductor which forms a capacitance at a tip end portion of the antenna. Capacitance plate 105 functions as a load capacitance of helical conductor 104, and flattens a frequency response of an input impedance of the chip antenna, so as to widen the frequency bandwidth. The use of the crown conductor is a common practice as illustrated in Japanese Patent Laid-open Publication Nos. 2002-124812 and H10-247806.
In a structure of the chip antenna illustrated in the publication H10-242731, however, the conductor to form a capacitance needs to be attached to the tip end portion of the antenna. This gives rise to a problem that the antenna becomes too large, especially for mounting, since it requires an increased number of component elements which make the structure complex and large. It also increases a number of production processes, and makes it difficult to produce at low cost. Because it is indispensable, especially for portable terminals and notebook computers, to be small and energy-saving, miniaturization of the chip antennas is thus desirable. However, the crown conductor attached to a tip end portion of any of a rod antenna and a pattern antenna was a problem because the entire antenna device becomes too large. It is quite undesirable to use the antenna devices so large in size, especially for cellular phones and notebook type computers, since they need to be made smaller and thinner to their limits.
There is also a problem when the antenna device, such as a rod antenna and a pattern antenna, bearing a crown conductor on the tip portion is mounted to a main circuit board, that a degree of flexibility decreases in selecting the shape of the crown conductor, or an area required for mounting the antenna device increases if more flexibility is given for the shape of the crown conductor. In addition, it is necessary to solve another problem, such as a loss of gain due to a position of the antenna with respect to the main circuit board.
The antenna device is built into a notebook type computer, a portable terminal, or the like. Some of examples are disclosed in Japanese Patent Laid-open Publication Nos. 2003-163521, H10-200438, H11-4117, and so forth. When the antenna device is to be mounted to an electronic apparatus, a mounting position is determined according to specifications of the apparatus. The mounting position is normally a top end of the apparatus, if it is a portable terminal or the like.
However, another problem was that the antenna device needs a large mounting area on the circuit board, which requires the circuit board to have an extra length to that extent. When the circuit board is made longer, an enclosure of the portable terminal also needs extra length to accommodate it, thereby making it difficult to reduce the size of the portable terminal.